1. Field of the Invention
This invention relates to an intermittently-fed rotary offset printing system of particular utility in printing identifying indicia on small, closely spaced articles, such as electronic components.
2. Description of the Prior Art
Known rotary article printing systems are generally continuously fed, by which it is meant that the articles to be marked are conveyed to the printing station at a uniform rate of speed. Printing is usually accomplished by a uniformly rotating die wheel or printing drum which makes rolling contact with the moving articles. Under these conditions, the delivery speed of the articles and the circumferential speed of the printing drum must be closely matched in order for a clear printed image to be obtained. A system for accomplishing this result is described, for example, in U.S. Pat. No. 2,761,545, to W. L. Hoagland.
In some cases, although the articles are delivered to the printing station at a uniform speed by a conveyor belt or the like, the spacing between successive articles may bear no convenient relationship to the spacing of printing dies on the rotary printing element. This may occur, for example, when the articles are delivered to the printing station in a widely or randomly spaced manner. In these instances, it has been the practice to drive the rotary printing element intermittently, stopping it at a known prealignment position after each printing operation and then accelerating it to the article speed when the arrival of the next article at a predetermined point is detected. Article marking systems operating in this general manner are disclosed, for example, in U.S. Pat. No. 3,092,019, to J. D. Van Buskirk, 3,394,651, to C. S. Ochs, and 3,738,260, to M. Navi et al.
A further problem arises where the spacing between successive articles approaches the closest spacing available between printing dies on the periphery of the printing drum. Electrical components, for example, are often fabricated in continuous strips with less than an inch between centers. It is inconvenient to provide the printing drum with the numerous identical, closely spaced printing dies that would be required under such conditions in a continuously-fed system, particularly where the dies are frequently changed to accommodate different types of articles. Moreover, even this expedient is foreclosed when the spacing between articles in smaller than the closest available die spacing, this being so regardless of whether the printing drum rotates uniformly or intermittently.
The above described limitations may be avoided by driving the printing drum uniformly and feeding the articles to the printing station intermittently rather than continuously. The geometry of the printing drum then imposes no limitation on the minimum spacing between successive articles.
U.S. Pat. No. 3,198,114, to F. Jones et al., describes such a system in connection with a printer for a perforated ticket strip. In this system, the transverse lines of perforations defining individual tickets on the strip are sensed by a photoelectric detector in order to halt the feed mechanism at a predetermined point of alignment. The printing drum continues to rotate uniformly, however. This is possible since there is no engagement between the drum and ticket surface until the single set of projecting printing dies thereon rotate to a position of frictional contact with the ticket surface. The angular position of the printing drum is sensed by a cam and follower switch arrangement which subsequently restarts the feed mechanism as the printing dies on the drum are about to frictionally contact the surface to be printed. By equalizing the peripheral speed of the drum and the linear speed of the ticket strip after this point, printing may be made to occur at the desired location on the ticket surface. Operation of the feed mechanism continues until the photoelectric detector encounters a further line of perforations, at which point feeding is again halted in preparation for another printing operation. The necessary intermittent feeding operation of the disclosed printing system is accomplished by a solenoid-displaced feed roller, or alternatively by an electromagnetic clutch and brake apparatus.
Adjustment of the position of the printed image on the ticket surface is accomplished mechanically in the printing system disclosed in the Jones et al. patent. The initial alignment position at which the ticket strip is halted prior to printing, for example, is adjusted by manually displacing the photoelectric detector along the feed table. Similarly, the angular position of the printing drum at which ticket feeding is recommenced for printing is manually set by rotating the cam follower mounting plate about the drum shaft and then securing it in the desired position with clamping screws.